In most of today's wind turbines the drive unit is placed in a nacelle, which also carries the wind turbine rotor. The nacelle is necessarily arranged on top of a high mast and rotates so that the rotor is always facing the wind.
Such placement of the drive unit results in a great weight at the top of the mast and that access for maintenance is a challenge. Yet it is this position of the drive unit and the generators that is used extensively as the alternative is that the rotational torque must be transmitted via a shaft through the mast, something that leads to losses and requires that the mast can absorb the reaction torque from the rotating shaft.
There is therefore an urgent need to simplify the drive unit and make it lighter. There is also a need to make access for maintenance easier. These are the main aims of the present invention, and these are achieved by the features that appear in the characterizing part of claim 1.
To use a belt to transmit the rotation from the rotor to a generator is known from, among others, WO 2008/028335 and JP 2005023893. However, the present invention aims to utilise the characteristics of the belt operation, or possibly the chain operation, better so that a more compact drive device can be achieved with additional operational benefits.
Some of the benefits that can be achieved by the invention in relation to the belt operation, according to the known solutions are:
One gets a doubling of the utilization of the capacity of the belts with respect to the known belt operation. This relates, in particular, to large (and therefore costly) belts and associated large belt wheels.
The structure becomes more compact.
The torque over the drive shaft is balanced.
The generation of power can be divided onto two generators that can be open to a more reasonable generator control system.
The time before one needs to replace belts can be made longer because one can drive the torque on a single generator when there is little wind.
In comparison to traditional drive systems with cogged wheel transfer, or direct operation, one can achieve the following benefits:
Reduction of shock loads (large momentum changes, vibration, etc. are dampened).
Makes complex lubrication and cooling systems unnecessary.
The system will be less prone to corrosion and have lower maintenance needs. This is particularly relevant for offshore wind turbines. The belt wheels can, for example, be given a zinc coating to reduce corrosion, something that is not possible with cogged wheel transmissions.
A weight saving is achieved by integrating the drive shaft in the belt wheel and also by taking up the torque over a large radius instead of a small radius inside a gearbox.
The number of parts that must be manufactured in the production of the drive system is significantly reduced.
A large part of the maintenance can be done without the use of a large crane. The belts are most susceptible to wear, but even the big belts do not weigh more than 70-100 kg. The belt wheel will not be subjected to wear as the belt is soft.
Scaling up to, for example, 5, 7 and 10 MW, will be possible without the mass of the drive system increasing exponentially. The mass of a 5 MW turbine with gearbox or direct drive will quickly become very heavy.
The power from the rotor can be distributed to several standard generators, something which results in increased flexibility.
A larger exchange can be brought about in one step than with cogged wheel transmission. This is because a large exchange in cogged wheel transmissions leads to high pressure forces on the cogs, something which results in much wear. With a belt operation, there will be a softer transmission and some slip may be acceptable at abrupt torque changes.
A belt drive system also has advantages compared with direct operation. With direct operation a relatively low rotational speed is transmitted to the generator. This means that the generator must be large and heavy. At higher rotational speeds one can use smaller and lighter generators.